The present invention relates generally to the construction of sewage manholes such as are utilized in municipal sewage and waste water handling systems and, more particularly, to a method and apparatus for rehabilitating deteriorated or otherwise defective sewage manholes.
In a typical municipal sewage handling system, an extensive network of underground pipes are provided to carry sewage and other waste waters to a municipal waste water treatment facility or other suitable treatment facility to purify the water for return to the municipal water supply. While the input of sewage per se into a municipality's water treatment system is generally predictable, the impact of fluctuations in rainfall and the resultant surface water run-off into a sewer system is highly unpredictable and, during periods of heavy rainfall, can overload a municipality's water treatment system. For example, it is not uncommon for a municipal treatment facility to receive on a rainy day two to three times the volume of sewage water received on a normal day without rain.
This problem of handling sewage water fluctuations due to intermittent rainfall can be significantly compounded by the presence in a municipality's sewer system of old deteriorating manhole structures and other manhole structures which may otherwise be defective and leaky because of faulty workmanship, settling, or other reasons. Local municipal ordinances and codes uniformly require the provision of manholes at periodic intervals along sewer lines to provide access into the sewer system when necessary to remove blockages, perform repairs, and correct other periodic problems. Generally, such manholes are subterranean structures which may be of a cast concrete construction or, in older manholes, may be of a brick and mortar construction. In either case, manholes should be sufficiently water-tight to prevent infiltration of ground water into the sewer system by seepage through the walls of the manhole. However, over time, the gases which are typically present in sewage, especially sulphide gas, gradually corrode and deteriorate mortar and concrete in manholes, to the point of ultimately permitting a substantial degree of ground water infiltration. Also, relatively new manholes can be subject to leakage and infiltration due to poor workmanship, earth shifting, or settling and like reasons. During periods of rainy weather, it is not uncommon for water infiltration through a single deteriorated or defective manhole to amount to ten to twenty gallons per hour and, thus, over an entire municipal sewer system, deteriorated and defective manholes can substantially contribute to overloading of a municipality's water treatment facility, the cost of which in turn is ultimately borne by the taxpayers.
Various means have been proposed for addressing the problem of deteriorated and defective manholes. The most obvious but most extreme and costly solution is to excavate, remove, and completely rebuild a deteriorated and defective manhole. A common alternative which is less extreme but still nevertheless costly is to construct a new water tight interior lining within the interior of an existing manhole's original wall structure. While these techniques can be generally effective for their intended purpose, the cost of implementing these solutions on a system-wide basis is viewed by many municipalities as being cost prohibitive. Accordingly, a substantial need exists for a simple and inexpensive yet effective means for rehabilitating deteriorated and defective manholes which will enable municipalities to minimize the strain imposed on water treatment facilities by ground water infiltration of manholes.